1. Field of the Invention
The present invention relates to a centrifugal fan, and more particularly, to a centrifugal fan which can reduce noise at air blowing.
2. Description of the Related Art
A centrifugal fan is configured by providing an impeller in a scroll casing. The impeller has a plurality of blades disposed around a rotation shaft of a motor, and the scroll casing has a suction opening and a discharge opening. Air suctioned from the suction opening flows from the center of the impeller into between the blades, and is discharged outward in the radial direction of the impeller by a fluid force due to a centrifugal action from the rotation of the impeller. The air discharged from the outer circumference of the impeller passes through the scroll casing to become high-pressure air, and is discharged from the discharge opening.
This centrifugal fan is widely used for cooling, ventilation, and air conditioning in home appliances, OA devices, and industrial equipment, an air blower for a vehicle, and the like. However, due to the configuration of the centrifugal fan, the air blowing performance and noise are significantly influenced by the blade shape of the impeller and the shape of the scroll casing.
Therefore, in order to reduce noise and improve air blowing performance, the optimization of the shape of the impeller and the configuration of the scroll casing has been attempted, and various proposals have been made. A centrifugal fan which optimizes a blade shape of an impeller to reduce noise has been proposed (see, for example, JP-A-S63-289295).
FIG. 11 is a plan view illustrating a centrifugal fan described in JP-A-S63-289295, and FIG. 12 is a perspective view illustrating an impeller of FIG. 11. The impeller 120 of the centrifugal fan includes a plurality of blades 123 installed between a main plate 121 and a sub plate 122 wherein the outer circumferential side of the blades 123 rotates with delay from the inner circumferential side of the blades 123 in the rotation direction of the impeller 120. A scroll casing 127 is attached to the impeller 120 to blow air.
The blown air is suctioned from the suction opening 140 of the impeller 120, discharged from the outer circumference due to the centrifugal action of the blades 123 of the impeller 120, guided to an air outlet 141 of the scroll casing 127 along the scroll casing 127 surrounding the outer circumference of the impeller 120, and discharged outside. In this blade configuration in which the outer circumferential side of the blades 123 rotates with delay from the inner circumferential side of the blades 123 in the rotation direction of the impeller 120, the blades are backward inclined blades and have a curved blade shape inclined backward in the rotation direction. The centrifugal fan having that blade shape is generally called a turbofan.
In the turbofan shown in FIG. 11, the plurality of blades 123 are interposed between the main plate 121 and the sub plate 122 having the same outside diameter, and the blades have tailing edges cut such that a blade arc 135 on the main plate side is shorter than a blade arc 134 on the sub plate side. Therefore, a time difference is generated between a time when each tailing edge 131 located on the main plate side crosses a tongue section 129 of the casing and a time when a corresponding tailing edge 132 located on the sub plate side crosses the tongue section 129 of the casing, such that pressure fluctuation occurring when the blades 123 cross the tongue section 129 of the casing is dispersed temporally, and sound generating energy is dispersed, so that generation of noise can be suppressed.
Recently, a noise reduction and a size reduction have been strongly demanded for centrifugal fans to be assembled in air blowers for home appliances, OA devices, and vehicles, and the like.
The turbofan described in JP-A-S63-289295 realizes suppression of noise when air is blown, by the shape of the blades 123. However, since it has a configuration in which air discharged from the outer circumference of the impeller flows along the inner wall surface of the scroll casing 127 and is discharged from the air outlet 141, disturbance of the air flow easily occurs in the vicinities of the inner wall surface and the air outlet 141 of the scroll casing 127, and the disturbance of the air flow causes noise.
Moreover, since the scroll casing 127 requires forming a flow path for guiding air to the air outlet 141 in the outer circumference of the impeller 120, the outside diameter of the scroll casing 127 needs a size of about two times the outside diameter of the impeller 120, and thus a reduction in size of the turbofan is difficult.